Ink jet printing processes are mainly of two types: continuous stream and drop-on-demand.
In continuous stream ink jet printing systems, a continuous ink stream is emitted under pressure through a nozzle. The stream breaks up into droplets at a certain distance from the nozzle. If a specific location on the recording sheet has to be printed the individual droplets are directed to the recording sheet, otherwise they are directed to a collecting vessel. This is done for example by charging unnecessary droplets in accordance with digital data signals and passing them through an static electric field which adjusts the trajectory of these droplets in order to direct them to the collecting vessel. The inverse procedure may also be used wherein uncharged droplets are collected in the vessel.
In the non-continuous process, or the so-called “drop-on-demand” systems, a droplet is generated and expelled from the nozzle in accordance with digital data signals only if a specific location on the recording sheet has to be printed.
The printing speed of modern ink jet printers is ever increasing for economical reasons. Recording sheets suitable for these printers therefore need to absorb the inks very quickly. Especially suitable are recording sheets containing nanocrystalline, nanoporous inorganic compounds, preferably oxides such as aluminum oxides or silicium dioxide, or oxide/hydroxides such as aluminum oxide/hydroxides.
Such recording sheets available today do not meet all of the required demands. In particular, the light stability and the storage stability of images printed on these recording sheets have to be improved. These images are not particularly stable, even in the dark, when they are in contact with ambient air, which normally contains sulfur dioxide and, especially in summer, photochemically generated impurities such as ozone and/or nitrogen oxides. The images are strongly altered or even destroyed in a short time when they are in contact with ambient air. These phenomena are described for example in “Ozone Problem with Epson Photo Paper”, Hardcopy Supplies Journal 6 (7), 35-36 (2000).
In patent application EP 0,685,345 the addition of dithiocarbamates, thiocyanates, thiurams or sterically hindered amines to recording sheets containing nanoporous inorganic oxides or oxide/hydroxides is proposed in order to improve the stability of printed images when these are in contact with polluted ambient air.
In patent application EP 1,197,345 the addition of unsubstituted or substituted 1,3-cyclohexanedione to recording sheets for ink jet printing containing nanoporous inorganic oxides or oxide/hydroxides is proposed in order to increase the stability of printed images when these are in contact with polluted ambient air.
In patent application EP 1,231,071 the addition of the salts copper(l) chloride, copper(l) bromide or copper(l) sulfite monohydrate to recording sheets for ink jet printing containing nanoporous inorganic oxides or oxide/hydroxides is proposed in order to increase the stability of printed images when these are in contact with polluted ambient air.
All these proposed additives however do not sufficiently increase the stability of printed images on recording sheets for ink jet printing containing nanocrystalline, nanoporous inorganic compounds when these are in contact with polluted ambient air. In particular, all reducing additives are quickly oxidized by oxygen or the impurities contained in the ambient air and therefore rapidly loose their stabilizing behavior. Some of the proposed additives may also be transformed into colored compounds when they are in contact with ambient air, leading to an unwanted degradation of the brightness of the recording sheets or of the images printed thereon.
Patent application DE 10,020,346 describes a recording sheet containing nanocrystalline, nanoporous silicium dioxide, where the primary particles have a size of not more than 20 nm, and proposes to modify the surface of the silicium dioxide with polyaluminum hydroxychloride in order to increase the stability of printed images when they are in contact with ambient air. The polyaluminum hydroxychloride may contain polynuclear aluminum hydroxo cations such as [Al6(OH)15]3+, [Al8(OH)20]4+, [Al13(OH)34]5+ or [Al21(OH)60]3+.
Patent application EP 1,437,228 describes a recording sheet containing nanocrystalline, nanoporous aluminum oxide or aluminum oxide/hydroxide and proposes to modify the surface of the aluminum oxide or aluminum oxide/hydroxide with aluminum chlorohydrate of formula Al2(OH)5Cl.2.5 H2O in order to increase the stability of printed images when they are in contact with ambient air.
The proposed methods of surface modification do not sufficiently increase the stability of printed images on recording sheets for ink jet printing containing nanocrystalline, nanoporous inorganic compounds when these are in contact with polluted ambient air. Furthermore, the reproducibility of the surface modification is not sufficient, because aluminum chlorohydrate as well as polyaluminum hydroxychloride are mixtures of not very well defined compounds. They also contain inactive ingredients.
In “Polyaluminum Chlorides” in “Kirk-Othmer Encyclopedia of Chemical Technology”, Volume 2, pages 338-345 (1992), ISBN 0-471-52670-3, the nature of the Al species in polyaluminum chloride products is discussed. This nature is not fully understood, but it appears that there are three or four species or molecular weight categories: monomers, a dimer, an Al13 polymer, and several higher Al polymers. The monomeric species Al(H2O)63+, Al(OH)(H2O)52+ and Al(OH)2(H2O)4+ seem to be known, as well as the dimer Al2(OH)2(H2O)84+ and the polymeric ion Al13O4(OH)24(H2O)127+. Characterization of aluminum chlorohydrate has revealed a predominance (about 88%) of Al13 polymer with the balance being monomers and smaller polycations.
P. M. Bertsch and D. R. Parker also discuss in “Aqueous Polynuclear Aluminum Species” in “The Environmental Chemistry of Aluminum”, Editor G. Sposito, CRC Press, second edition, pages 117-168 (1996), ISBN 1-56670-030-2, the nature of polynuclear hydrolytic aluminum species. They state that despite several decades of active research, considerable disagreement concerning which of various polynuclear hydroxo-Al species are truly significant persists.
The polynuclear aluminum hydroxo complex Al30 having a polycation with 30 aluminum atoms has been described by G. Allouche, C. Gérardin, T. Loiseau, G. Féray and F. Taulelle in “Al30: A Giant Aluminium Polycation”, Angewandte Chemie 112, 521-524 (2000). They assign to Al30 the formula [Al30O8(OH)56(H2O)24]18+.
The polynuclear aluminum hydroxo complex Al30 having a polycation with 30 aluminum atoms has also been described by J. Rowsell and L. F. Lazar in “Speciation and Thermal Transformation in Alumina Sols: Structures of the Polyhydroxyoxoaluminum Cluster [Al30O8(OH)56(H2O)26]18+ and its δ-Keggin Moieté”, Journal of the American Chemical Society 122, 3777-3778 (2000). They assign to Al30 the formula [Al30O8(OH)56(H2O)26]18+. The authors declare: “Despite over 50 years of research, only one ubiquitous polyoxocation has ever been unambiguously characterized, namely the hydroxo complex with 13 aluminum atoms. Only circumstantial evidence exists for other species.”
There is therefore still a need to improve, in addition to the ink absorption capacity, the image quality, the water fastness, the light stability etc. of recording sheets containing nanocrystalline, nanoporous inorganic compounds, in particular the storage stability of images printed onto such recording sheets when they are in contact with ambient air containing impurities such as ozone, nitrogen oxides or sulfur dioxide.